Overview
Producing high optical quality and a smooth sagging profile for windshields requires precise process control, especially given modern design demands like large size, deep sagging, asymmetric glass thickness, colored float glass, coated glass, special glass types, extensive printing, and small installation angles. These factors create challenges in the bending process, particularly in balancing temperature distribution.
Key Design Challenges in Bending
- Asymmetric Glass Thickness: Common combinations, like 2.1 mm and 1.6 mm, or other thin <1.8 mm glass plies, demand careful IR heating control to prevent temperature imbalances that can cause optical distortions.
- Colored Float Glass: Darker glass, such as green float glass, absorbs more heat, complicating uniform heating and risking overheating in thinner sheets.
- Low Installation Angle: Lower angles magnify any optical defects, as light travels through more glass.
- Complex Geometry: Larger windshields with deep curvature (>30 mm) increase the likelihood of optical distortion due to uneven sagging profile
- Larger Printed Areas: Printed areas absorb more heat, potentially leading to “areal overheating” and distortions in the sagging profile.
- Special Glass Types: different type of special glass types such as borosilicate, and aluminosilicate, or others react differently to IR-heating and can require different type of heating profile approaches. Especially in case of hybrid formulations where special glass type is combined together with soda lime silica float glass.
Temperature Balance and Quality Control
Temperature differences between glass sheets often arise from:
- Main Heating Source Position: Roof-mounted IR heaters primarily heat the top glass sheet (short glass / inner glass), leading potentially to overheating.
- Glass Color and Thickness: Thinner or darker sheets absorb IR-heating faster, necessitating careful bottom heating to maintain temperature balance.
- IR Penetration Depth: IR-heating heats only the very surface of the glass, while majority of the glass is heated through conduction, making bottom heating crucial for asymmetric glass combinations.
When top sheet (inner glass) overheat, they can sag against the bottom sheet, imprinting surface imperfections, hence potentially causing optical defects. Optimum temperature balance is maintained with uniform heating from both directions, and may require advanced tooling solutions or furnace adjustments.
Producing Superior Optical Quality
- Uniform Heating: Ensure balanced heating throughout, with specific attention to bottom heating to counteract roof heaters. Mold height and furnace technology should support uniform IR-heat distribution through both sheets.
- Smooth Sagging Profile: A continuous, distortion-free sagging profile requires customized heating configurations and mold tooling to counteract natural tendencies for flat or reverse bending. Heat mirrors (a.k.a. sombreros) or absorber plates can help redirect heat away from over-heating areas such as the printed areas.
*When applying additional mold tooling solutions such as heat mirrors or heat absorbtions plates pay attention to residual stresses as these solutions potentially effect compression and tension levels. Especially if not applied correctly.
Conclusion
Superior optical quality in windshields results from precise heating control, accurate temperature monitoring, and tailored mold ( skeleton ) tooling solutions or adjustments. Temperature data logging with Datapaq or Phoenix type dataloggers can significantly improve trouble shooting, and new model introduction providing temperature data to drive correct heating recipe and tooling adjustments required optimal optical quality.